A dental composition containing a polymerizable monomer, a filler, and a polymerization initiator is called a composite resin, and this dental composition is most widely used today as a restorative material for repairing fractures of teeth and dental caries. Such a dental composition is required to have the following properties. Specifically, as a cured product obtained after polymerization curing, the dental composition is required to have sufficient mechanical strength and hardness to serve as a substitute for natural teeth, wear resistance against occlusion of teeth in an oral cavity, surface smoothness and gloss, color matching with natural teeth, transparency, etc. Furthermore, as a paste which has not yet been polymerized and cured, the dental composition is desired to have ease of handling (high handling properties) for dental clinicians and technicians, for example, proper fluidity and forming property, no adhesion to dental instruments, no stickiness, etc.
These properties of the dental composition are greatly influenced by the component materials, shape, particle size, and content of fillers used therein and by the combination of the fillers used together. For example, when an inorganic filler having an average particle size of more than 1 μm is used, the filling rate of the filler in the polymerizable monomer can be increased easily and therefore sufficient mechanical strength as a cured product and high handling properties as a paste can be obtained. The use of such an inorganic filler has, however, a drawback in that it is difficult to obtain satisfactory gloss even after final polishing, and even if satisfactory gloss is obtained, the gloss cannot be retained for a long time. On the other hand, when an inorganic ultrafine particle filler having an average particle size of 1 μm or less is used, the surface smoothness and gloss after polishing of the cured product and the gloss durability in the oral cavity are improved. The use of such an inorganic ultrafine particle filler has, however, a drawback in that when the inorganic filler is mixed and kneaded with the polymerizable monomer, the viscosity of the resulting paste increases significantly, which makes it difficult to increase the content of the filler. As a result, the mechanical strength of the cured product decreases, and the unpolymerized pasty composition becomes sticky, which reduces the handling properties. Furthermore, when an organic-inorganic composite filler obtained by mixing inorganic ultrafine particles having an average particle size of 100 nm or less with a polymerizable monomer, curing the mixture, and grinding the resulting cured product is used, the handling properties of the paste are improved, but the content of the inorganic filler in the cured product still is insufficient. Since the surface of the organic-inorganic composite filler forms a weak bond with the matrix, the mechanical strength of the cured product is not sufficient. Under these circumstances, it is difficult to increase the mechanical strength and the surface smoothness and gloss after polishing of the cured product and the handling properties of the paste in a balanced manner.
In recent development of dental compositions, various methods have been proposed to solve conventional problems such as the stickiness of pastes and insufficient mechanical strength while inorganic ultrafine particles are used as a main component to ensure the surface smoothness and gloss after polishing. For example, Patent Literature 1 listed below describes a dental composite resin containing an organic-inorganic composite filler having an average particle size of 1 to 30 μm and a glass powder having a particle size range of 0.1 to 100 μm and an average particle size of 0.2 to 20 μm. The organic-inorganic composite filler is obtained by mixing ultrafine particle silica having an average particle size of 0.01 to 0.05 μm with a polymerizable monomer, polymerizing and curing the mixture, and grinding the resulting cured product.
Patent Literature 2 discloses a technique of using, as a filler, an aggregate having a primary particle size of 1 to 250 nm and obtained by heat treatment of silica and at least one metal oxide other than silica. For example, Patent Literature 2 discloses a dental composition using, as a filler, an aggregate of silica-based fine particles and zirconia fine particles, obtained by mixing a silica sol having an average particle size of 15 nm and a zirconia sol having an average particle size of 23 nm, and subjecting the mixture to spray drying and heat treatment.
Citation List
Patent Literature
Patent Literature 1 JP 63 (1988)-88110 A
Patent Literature 2 JP 2001-302429 A